Today, a plurality of contiguous networks may be grouped in a large entity called an Autonomous System (AS). The AS is under a common administration that shares a common routing strategy managed by the OSPF (Open Shortest Path First) protocol. OSPF is a link-state routing protocol that calls for sending of link-state advertisements (LSA) to all other routers within a particular area. Such LSAs include information on attached interfaces, metrics being used and other variables.
An AS is generally divided into a number of areas which are groups of contiguous networks and attached hosts. Routers with multiple interfaces can participate in multiple areas; such routers are called area border routers. Each router maintains a data base describing the AS topology. A topological data base is essentially an overall picture of networks in relationship to routers. The topological data base contains the collection of LSAs received from all routers in the same area. Because the routers within the same area share the same information, they have identical topological data bases. Each individual piece of a topological data base is a particular router's local state (e.g. the router's usable interfaces and reachable neighbors).
All routers of a same area run the same algorithm in parallel. From its topological data base, each router constructs a tree of shortest paths with itself as a root. This shortest path tree gives the route to each destination in the AS.
The topology of an area is hidden from the rest of the AS. This information hiding enables a significant reduction in routing traffic. Also, the routing within the area is determined only by the area's own topology lending the area protection from bad routing data. Keeping area topologies separate, OSPF protocol passes less routing traffic than it would pass if the AS was not partitioned. Furthermore, this partitioning creates two different types of OSPF routing, depending on whether the source and destination are in the same areas or are in different areas.
The shortest path first (SPF) routing algorithm is the basis for OSPF operations. After a router is assured that its interfaces are operating, it uses the OSPF Hello protocol to acquire neighbors which are routers with interfaces to a common network. The router sends hello packets to its neighbors and receives their hello packets. In addition to helping acquire neighbors, hello packets also act as means to let routers know what other routers are still functioning.
Among the different areas of the AS, an OSPF backbone (or area 0) is responsible for distributing routing information between areas. As the backbone itself is an OSPF area, all backbone routers use the same procedures and algorithms to maintain routing information within the backbone as the routers of any other area. The backbone topology is invisible to all routers within the other areas.
Stability and redundancy are the most important criteria for the backbone. Stability is increased by keeping the backbone size reasonable. Insofar as every router in the backbone needs to re-compute its routes after every link-state change, keeping the backbone small reduces the likelihood of a change and reduces the amount of CPU cycles required to re-compute the routes.
The main issue when implementing a multi-area OSPF Autonomous System is to have a very reliable backbone since all communications are transmitted through the backbone which, therefore, must be available all the time. A solution to this problem is to duplicate nodes and links, and in particular the routers connecting an area to the backbone called Autonomous System Border Routers (ASBR), which minimizes the risk of an area from becoming disconnected from the backbone. However, such a duplication is not sufficient in view of well known OSPF storms as well as disruptive software upgrades which may prevent the backbone infrastructure from being used part of the time.